Digital three-dimensional object manufacturing, also known as digital additive object manufacturing, is a process of making a three-dimensional solid object of virtually any shape from a digital model. Three-dimensional object printing is an additive process in which one or more ejector heads eject successive layers of material having different shapes on a substrate. Typically, ejector heads, which are similar to printheads in document printers, include an array of ejectors. Ejectors within a single ejector head can be coupled to different sources of material or an ejector head can be coupled to a single source of material to enable all of the ejectors in the ejector head to eject drops of the same material. Materials that become part of the object being produced are called build materials, while materials that are used to provide structural support for object formation, but are later removed from the object are known as support materials. Three-dimensional object printing is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling.
A previously known three-dimensional object printing system 10 is shown in FIG. 10. In the view depicted in that figure, a platform 14, called a cart, includes surfaces 18 (FIG. 9) that slide upon the upper surfaces of track rails 22 to enable the cart to move in a process direction P between printing stations, such as the printing station 26 shown in FIG. 10. Alternatively, carts can include wheels configured to roll along tracks, or other types of acceptable mobility mechanisms. Printing station 26 includes four ejector heads 30 as shown in the figure, although fewer or more ejector heads can be used in a printing station. Once the cart 14 reaches the printing station 26, the cart 14 transitions to and moves along precision rails 38 through the printing station. Precision rails 38 are cylindrical rail sections that are manufactured within tight tolerances to help ensure accurate placement and maneuvering of the cart 14 beneath the ejector heads 30. Linear electrical motors are provided within housing 42 of the track to interact with a magnet inside housing 46, which is connected to the lower surface of the cart 14. The motors generate electromagnetic fields that interact with the magnet to propel the cart along the track rails 22 between print stations and along the precision rails 38 within the printing stations. Once the cart 14 is beneath the printing station 26, ejection of material occurs in synchronization with the motion of the cart. Electrical motors (not shown) are operatively connected to a gantry to which the ejector heads are mounted to move the ejector heads in an X-Y plane that is parallel to an upper surface of the cart 14 as layers of material are formed in the object. Once the printing to be performed by a printing station is finished, the cart 14 is moved to another printing station for further part formation, layer curing, or other processing.
An end view of the system 10 is shown in FIG. 9. That view depicts in more detail the surfaces 18 that rest upon the rails 22 that extend from and above the electrical motor housing 42 of the track. As the motors generate electromagnetic fields that interact with the magnet in housing 46, the surfaces 18 of the cart 14 slide along the track rails 22. At the printing station, the bearings 34 of the cart 14 contact the precision rails 38 in an arrangement that facilitates accurate positioning of the build platen on the cart 14. Specifically, a pair of bearings 34 on one side of the cart 14 are positioned at a right angle to one another on one of the rails 38 to remove four degrees of freedom of the cart 14, while the bearing 34 on the other side of the cart 14 rests on the other rail 38 to remove one more degree of freedom. Gravity and magnetic attraction between the electrical motor and the magnet in the housing 46 hold the bearings 34 in contact with the rails 38.
Additional motors (not shown) move the printing station 26 vertically with respect to the cart 14 as layers of material accumulate to form an object. As material for forming the object accumulates, the topmost surface of the object grows closer to the ejector heads 30. Accurate ejection of material generally requires accurate control of a distance between the ejector head 30 and the surface receiving ejected material. Further, higher surfaces of an object being formed may impact the ejector heads 30 or another printing station 26 unless the distance between such stations and the top of the object is accurately controlled.
Generally, such additional motors for vertically moving the ejector heads 30 are configured to index a vertical location of the ejector heads to raise and lower the ejector heads to a height corresponding to a surface onto which material is to be ejected. However, even the diminutive motion needed to raise an ejector head 30 to account for an accumulated layer of material on a surface can affect the location of the ejector head 30 with regard to the object being formed, and negatively impact the accuracy of the printed object. Further, printing stations 26 configured with movable gantries that enable vertical indexing of the ejector heads 30 are complex, and require significant upkeep and testing to ensure accurate printing. Additionally, indexing the ejector heads 30 upwards requires all print jobs in the printing system 10 to simultaneously be on a same level of thickness, since indexing the ejector heads 30 for one cart 14 would require that the ejector heads 30 be subsequently relocated vertically for each additional cart. Therefore, a system for vertically indexing mobile carts that move through a printing system to enable accurate printing of three-dimensional objects with ejectors that are stationary in the Z-axis would be beneficial.